1. Field of Invention
It has long been recognized that warming intravenous fluids to body temperature is beneficial and for rapid infusions of blood or other cold fluids such warming is necessary to prevent cardiac arrhythmias and possible cardiac arrest.
Fluid warmers have several challenges to meet. First, blood must not be overheated, or lysis of red cells occurs making the infusion toxic. Second, high flow rates are sometimes needed to replace blood volume in the event of rapid surgical blood loss. Most surgical cases, however, use only 1 to 2 liters of intravenous fluids over one or more hours, at low flow rates. Some studies have shown that warming fluids for these low flow rate cases is beneficial, and for pediatric cases, warming fluids is important for infusions that are large relative to the patient's weight even though absolute flow rates are low. Only a few blood warmers can meet the challenge of high flow rates (up to 500 ml/min), and most blood warmers can only effectively warm fluid at the low flow range down to about 25 ml/min.
The present invention provides normothermic fluid to the patient at very low flow rates, essentially down to zero milliliters per minute. Applied to my previous invention, U.S. Pat. No. 6,608,968 B2, normothermic flow rates of zero to 600 milliliters per minute are achieved. Further, it provides this superior performance using a single, low cost disposable. In combination with other in-line blood warmers, the disposable patient line of this invention would extend their performance to deliver normothermic fluid to the patient for low flow rates down to essentially zero milliliters per minute.
2. Description of Prior Art
My previous U.S. Pat. No. 6,608,968 B2 described a convection blood warming system with disposable flattened tube envelope incorporating paperboard “needle” for inserting envelope between heating plates and employing active and passive insulation of the outlet flow path to provide normothermic fluid at zero to 600 milliliters per minute. The active insulation of the outlet flow path of that invention was an optional, reusable external heater applied to the distal portion of the patient intravenous line.
My previous U.S. Pat. No. 5,420,962 related to a disposable system that incorporated a hydrophobic vent patch into the disposable envelope heat exchanger. It also provided for preservation of heat in the patient i.v. line by passing the i.v. line through a larger diameter (about 1 inch dia) flexible corrugated plastic tubing. Warm air was passed through the outer tube, bathing the i.v. line and reducing the heat loss to the ambient air.
U.S. Pat. No. 5,875,282 employs a flat envelope heat exchanger carried into position by a rigid plastic cassette and warms the heating plates directly with resistance heaters, but is less effective at high and low flow rates.
U.S. Pat. No. 5,063,994 utilizes a patient line with a central intravenous fluid lumen surrounded by a warm water carrying annular lumen which is divided in half. The warm water flows toward the patient in one half of the annular lumen, turns around 180 degrees and returns to the blood warmer, actively warming the intravenous fluid, but is only effective at low flow rates.
The present invention provides superior low flow rate performance by employing an active, low cost, temperature-controlled electrical resistance heater in the annular air space of the disposable patient intravenous line to maintain the intravenous fluid at normothermic temperature at low or zero flow rates. For pediatric or other extremely low flow uses, the extremely low flow rate performance allowed by this active warming of the distal line is very important. Including an economical active warming heater to the disposable is very desirable because it is so convenient.